Typically, sheet-conveying drums are located between the printing units and between the further-processing units, such as varnishing works, of a rotary printing machine. A sheet-conveying drum generally has one or more gripper bridges that are distributed evenly about the circumferential surface of the drum. These gripper bridges hold the sheets of paper to the drum enabling the drum to convey the sheets from the previous unit to the next unit downstream in the printing machine. In order to accommodate sheets of different thickness, the gripper bridges have adjustable gripper impact strips.
A sheet-conveying drum of this type is known from DE 3,428,668 C2, in which the drum has a central adjustment mechanism for adjusting the gripper pads in each of the gripper bridges to accommodate sheets of different thickness. The central adjustment mechanism is arranged on the drum body and acts on a positioning mechanism which acts on means that simultaneously adjusts all of the gripper pads. Further, the central adjustment mechanism can be driven by a motor and controlled from a control panel. However, the central adjustment mechanism is not capable of separately adjusting each individual gripper pad. An individual adjustment of each gripper impact strip can only be accomplished by releasing the retained central adjustment mechanism and manually adjusting the individual gripper impact strips. Since it is not easy to individually adjust each gripper pad, inaccuracies in sheet receiving or sheet transferring between sheet transfer cylinders cannot be effectively remedied. This could lead to slipping or tearing of the sheets which would disrupt the registry of the sheets resulting in lower quality printing. In addition, adjustment of the gripper pads is quite complicated and time consuming, requiring both a basic adjustment step and a fine adjustment step. This leads to increased production down time, since the printing press must be shut down in order to adjust the gripper pads for a new sheet thickness.
GB 2,098,966 A discloses a sheet-conveying cylinder having a mechanism for adjusting the gripper pads in a gripper bridge for sheets of different thickness. The gripper pads for the gripper bridge are mounted on a gripper pad bar which extends axially to the cylinder. The mechanism comprises an adjusting rod which is arranged radially inward of the gripper pad bar and in contacting relation therewith and a plurality of adjustment screws that threadedly extend through the adjustment bar. The contact surfaces of the adjustment bar and the gripper pad bar are complimentarily slanted in the transverse direction of the bars. Angular movement of the adjustment screws causes the adjustment bar to move thereby displacing the gripper pad bar radially inward or outward to accommodate a different sheet thickness. This type of adjustment mechanism is undesirable because a significant amount of time is required to adjust the gripper bridge for a different sheet thickness since a plurality of screws must manually adjusted.